In modern sophisticated medical procedures there is the necessity for measuring pressures of a variety of fluids. Thus in the practice of orthopedic surgery, there is the necessity for injecting polymerizing plastic materials under pressure into cavities surrounding prosthetic devices inserted within femurs so as to affix them in the desired positions. If such polymerizing plastic materials are subjected to insufficient pressure, there arises the danger of incomplete injection. On the other hand, if the pressure is excessive, there arises the danger of excessive penetration of medullary bone. Moreover, it has been found desirable to subject the injected polymerizing plastic materials to successive levels of increasing pressure; it is also desirable to delay the injection of the cement into the canal until the cement is in a moderately advanced stage of polymerization and delay the initial exposure of cement to saline, which is in blood and serum, for as long as possible. When saline is mixed with cement in the early stages of polymerization, that cement when hardened or mature will be brittle as compared with cement which was in a more advanced stage of polymerization when it first made contact with saline. Furthermore, the surface area of the cement which is initially exposed to saline should be at a minimum; minimal surface area is obtained if the cement is not driven deeply into the small medullary spaces or intertrabecular spaces. And so, if the injection of the cement into the canal is delayed until the cement is in a moderately advanced stage of polymerization (i.e., until the cement is near the dough stage), it will not penetrate intermedullary spaces. Because the use of a closed system of pressurization will be employed, the surgeon is assured that desired pressurization and penetration will be achieved.
The insertion of the prosthetic stem into the canal and cement should be a slow process so as to allow the cement in the distal end of the canal to move upward by exerting a minimum amount of pressure of less than 15 lbs/sq. inch and so the cement continues to polymerize with minimum penetration into intertrabecular spaces and with minimum surface area exposure of the cement to saline. Without a closed system of pressurization, the insertion of the prosthetic stem into the cement in the canal is done rapidly which creates a pressure of 18-20 lbs/sq inch and so the cement is driven upward toward the proximal end of the femur but is also driven laterally into the medullary spaces thereby increasing the surface area of the cement to saline at an earlier stage or less polymerized stage of the cement.
The pressures stated herein are average range of pressures which the surgeon might choose, the pressures are for the commonly used cements, namely, Howmedica's Surgical Simplex P and Zimmer's L.V.C. (low viscosity cement) for patients with systolic blood pressures of 120 mm to 150 mm of mercury. The pressures would be different for other cements; and would be different if the ratio of monomer (liquid) and polymer (powder) were to be changed.
The bleeding into the femoral canal will be stopped with 15 lbs/sq. inch if the patient's systolic blood pressure is 150 mm of mercury or stopped with 12 lbs/sq. inch if the systolic pressure is 120 mm of mercury.
When the definitive prosthetic stem with the closed system of pressuring apparatus is secured to the femur, additional cement is injected to completely fill the proximal femoral canal with a pressure of approximately 12 to 15 lbs./sq. inch which will produce hemostasis of blood for the entire cement cavity, eliminate air from the cemented area but without deep penetration of the cement into the medullary spaces. One may then delay further pressurization and allow more advanced polymerization of the cement for about one minute, then pressurize to 23 or 24 lbs./sq. inch; thereby filling the medullary spaces with a high grade of cement with minimum mixing with saline. At still approximately one minute later pressurize to 35-37 lbs./sq. inch and immediately close the inlet valve. The clearing of the cement from the inlet stem will raise the pressure 5 to 8 lbs./sq. inch with a resultant final pressure of 40-43 lbs./sq. inch.
As will be evident to one skilled in the medical arts, it is not only desirable to accurately measure and control pressure levels for the foregoing procedures, but it is also necessary to maintain integrity of the injected materials and to prevent their contamination. Moreover, since such materials solidify and become difficult to remove, it is necessary to prevent their entry into pressure measuring apparatus where they could plug up or otherwise interfere with operation.
In the past, a variety of proposals have been made to achieve the foregoing objectives. Thus, for example, cylindrical plugs often fitted out with "O" rings have been inserted into cylindrical passageways leading from the material to the gauge. Such cylindrical plugs prevent the materials from entering the gauge but have been found to be impractical with polymerizing substances. According to other proposals, a line leading from the material to the gauge has been filled with a compatible liquid. However, such a fluid filled line, with or without "O" rings permits some contamination of the material; and with polymerizing materials the line is soon plugged and the assembly becomes difficult to disassemble and service.
Still other proposals have been made in the past. Thus, transducers have been used in contact with polymerizing materials, but such transducers often require that the exposed tip of the transducer in contact with the material be covered with wax or petroleum jelly which creates a degree of objectionable contamination. Moreover, such transducers ordinarily require the connection of electrical wires leading from the material being tested to an electronic panel which receives, converts and records the impulses from the tip of the transducer insert, with the result that the equipment crowds an already busy and limited working field such as an operating table and/or room. Accordingly, there has continued to be a need for an improved pressure measuring method and apparatus.